Get all your ingredients measured and ready. Slice butter into 1 tablespoon chunks and keep in the refrigerator. Zest and juice the lemons; you'll need about 1/4 cup zest and 1/3 cup juice.

In a double boiler, whisk together egg yolks and sugar until combined. Add lemon zest and juice, whisking continuously over medium heat for about 8-9 minutes until the mixture begins to thicken.

Remove from heat. Add cold butter one tablespoon at a time, whisking until melted and smooth. Transfer to a bowl, cover, and refrigerate for at least 2 hours until chilled and thickened.